A PLL circuit is an electrical system with electrical elements arranged in a loop. The electrical elements of a PLL circuit can include a phase frequency detector (PFD), a charge pump (CP), a loop filter (LF), a bias generator (BG), and a voltage controlled oscillator (VCO). These electrical elements can be connected on a circuit pathway that is a PLL.
In the loop of a PLL circuit, an output signal from an electrical element in the loop can be an input signal for a next electrical element in the loop. Thus, in a PLL, an output signal from a PFD can be an input signal for a CP, an output signal from the CP can be an input signal for a LF, an output signal from the LF can be an input signal for a BG, an output signal from the BG can be an input signal for a VCO, and an output signal from the VCO can be an input signal for the PFD. The signal from the VCO to the PFD is considered to be a feedback signal. In addition to the feedback signal, the PFD can also receive a reference signal from outside the loop of the PLL circuit.
A PLL circuit can lock the feedback signal to the reference signal. PLL locking can include frequency locking and phase locking. Frequency locking can include adjusting a frequency of the feedback signal to correspond with a frequency of the reference signal. Once this frequency locking adjustment is complete the PLL is considered to be frequency locked. Phase locking can include adjusting a phase of the feedback signal into a particular fixed phase relationship with the reference signal. Once this phase locking adjustment is complete the PLL is considered to be phase locked. Throughout this document, a PLL circuit that is frequency locked and phase locked is considered to be locked.
A delay locked loop (DLL) is also an electrical system with electrical elements arranged in a loop. The electrical elements of a DLL circuit can include a PD, a CP, a LF, a BG, and a voltage controlled delay line (VCDL). These electrical elements can be connected on a circuit pathway that is a DLL.
In the loop of a DLL circuit, an output signal from an electrical element in the loop can be an input signal for a next electrical element in the loop. Thus, in a DLL, an output signal from a PD can be an input signal for a CP, an output signal from the CP can be an input signal for a LF, an output signal from the LF can be an input signal for a BG, an output signal from the BG can be an input signal for a VCDL, and an output signal from the VCDL can be an input signal for the PD. The signal from the VCDL to the PD is considered to be a feedback signal. In addition to the feedback signal, the PD can also receive a reference signal from outside the loop of the DLL circuit. A DLL circuit can phase lock the feedback signal to the reference signal. Throughout this document, a DLL circuit that is phase locked is considered to be locked.
PLL circuits are widely used in many applications. For example, in one application, PLL circuits are used as multi-phase clock generators that generate a plurality of clock signals phase-shifted in equally-spaced increments relative to a reference clock signal. PLL multi-phase clock generators are useful in electronic systems having complex timing requirements in which multi-function operations are completed during a single reference clock cycle or in which an operation extends over more than one reference clock cycle. In another application, PLL circuits are used as frequency multipliers that generate a clock signal having a higher frequency than the frequency of the reference clock signal.
Conventional PLL circuits have characteristically slow lock times. Lock time is the time required to lock. In particular, conventional PLL circuits lock both phases and frequencies in order to lock. This causes conventional PLL circuits to have increased lock time relative to other clock synchronization circuits (e.g., DLL circuits) that simply lock phases in order to lock. The relatively slow lock times of conventional PLL circuits are also attributable to the inability of conventional PLL circuits to adjust phase without simultaneously adjusting the frequency. Nevertheless, PLL circuits are useful in various applications, such as generating clock signals, recovering clock data, and synthesizing frequencies, as will be understood by one of ordinary skill in the art.